What if there were no lasers today?

When you hear the word, “laser” what are you reminded of? Luke Skywalker? Star Wars? High-tech wars between spacecraft?

Well, those concepts make good movies and TV shows, but they don’t make very good sense - in a practical way. In the last 40+ years, we have created a wide range of lasers (some whose output you can’t even see) and we’ve learned how to control them and use them to make our life better and to do things we’ve never been able to do with any other device - incredible breakthroughs in medicine, communications, manufacturing, entertainment and lots more. Unless we happen to be involved in the development of some application of the laser we probably don’t even know they are being used - right before our eyes!

Lasers now come in a variety of configurations and output wavelengths (colors), in continuous and pulsed beams, and at high and low power levels. We can often find a “laser solution” to a particular problem by selecting a laser with an output that suits our needs best. The unique properties of lasers that make them useful are:

• Monochromatic - Most lasers emit a beam of light at a very pure color (or wavelength). This means that the beam will be selectively transmitted, absorbed or reflected when other beams of light are not affected the same way.


• Collimated - A laser ray can be made to remain a very narrow beam that will travel long distances without spreading out much. A laser beam can be sent all the way to the moon and spread so little that it still makes a powerful spot when it hits something.


• A powerful Source of Heat that can be directed and pin pointed to an exact spot where it may melt or vaporize the target material, and yet leave the surrounding material unaffected.
  
• Coherent - Because laser light is much better organized than ordinary light, lasers have the same “information-carrying” properties that radio waves have, except the laser is working at much, much higher frequencies. This allows huge amounts of information, and many, many channels to be sent over a laser beam. Sometimes the laser beam is sent in the air; and sometimes it is “piped” in tiny plastic or glass strands called “fiber optics”.

So what are some common uses of lasers that we use every day? Here are a few:

Supermarket Checkout Systems
A low-power laser beam is scanned across the “bar codes” that are attached to products we buy. When we check out at a superstore, we just place the product with its bar code face down on the window of the scanner, the laser beam sweeps across the bar code and the reflected laser beam is read as a code that identifies the product. This uses the collimated and monochromatic characteristics of the laser.

LASIK Eye Surgery
LASIK (laser-assisted in situ keratomileusis) is a surgical procedure that uses a laser to correct nearsightedness, farsightedness, and/or astigmatism. In LASIK, a thin flap in the cornea is created using a femtosecond laser. The surgeon folds back the flap, and then removes some corneal tissue underneath using an excimer laser. The flap is then laid back in place, covering the area where the corneal tissue was removed. With nearsighted people, the goal of LASIK is to flatten the too-steep cornea; with farsighted people, a steeper cornea is desired. LASIK can also correct astigmatism by smoothing an irregular cornea into a more normal shape. This application uses the collimated, monochromatic and heat properties of the laser. (Unfortunately, laser pioneers are too old to be considered good candidates for LASIK.)

Laser Printers & Copiers
The physical phenomenon at work in a laser printer is static electricity, the same energy that makes clothes in the dryer stick together. A laser printer uses this phenomenon as a sort of "temporary glue" to hold toner on a photoconductive drum. The laser "writes" the print information on a photoconductive revolving drum, which then transfers it to a sheet of paper. This uses the collimated and heat properties of the laser. The information is then sealed to the paper with heat from a fuser, producing a very high-resolution copy.
(From www.howstuffworks.com/laserprinter.htm )

There are more laser applications to talk about (internet, displays, entertainment, pointers, and defense/homeland security equipment); but, those will have to wait until there’s another blog posting.

Questions or comments? Post your comments here or e-mail me!

Black Silicon for Solar Cells: What does it mean?

Silicon is the “material of choice” for solar voltaic cells because it takes relatively little energy to excite the electrons in a silicon crystal into the conduction band, where they are free to conduct electricity that can be used to generate electric power. Scientists and engineers have spent many years trying to modify silicon crystals by doping them with other materials that would make the photovoltaic process even more efficient. In the last few years, scientists at Harvard University found that if they blasted the surface of a silicon wafer (in the presence of sulfur hexafluoride gas) with femtosecond laser pulses, the silicon received a heavy doping of sulfur and the surface of the silicon developed deep, microscopic cones. The result was that this newly formed, thin silicon layer, called “black silicon” is 200-500 times more sensitive to light than untreated silicon. How is this possible?

With its new structure, the band gap in the thin silicon layer - the difference between the valence band and the conduction band - is smaller. This means that longer wavelengths of sunlight (infrared) are also able to excite electrons into the conduction band - contributing energy to the solar-electric conversion. Furthermore, by applying a small voltage (a bias) to black silicon creates conditions in which each incoming photon can excite still more electrons. So, not only is the material responsive to wavelengths that silicon-based devices couldn’t detect in the past - it also produces a much stronger signal in response to a weak stimulus. The increased sensitivity makes black silicon good for detection applications; and the increase in absorption wavelengths improves the efficiency of silicon for solar cells.

The Harvard Scientists (Stephen Saylor and James Carey) formed a spinoff company, SiOnyx, to commercialize the process for solar energy and highly-sensitive, imaging applications, such as night vision, surveillance, digital cameras and medical imaging. For over three years, they’ve been pretty quiet about this, but in the last few months, they are starting to talk. Maybe that’s a sign that some of the applications of black silicon are about to “come to market”.

This is another example of photonics (femtosecond lasers) as an enabling technology - this time to enhance the efficiencies of solar voltaic cells.

To read the complete article about black silicon, go to:

www.xconomy.com/boston/2008/10/12/sionyx

What do you think about the potential of “black silicon” for solar cell improvement? What other breakthroughs do you see in solar electric energy?